U.S. patent application number 11/627485 was filed with the patent office on 2007-08-09 for solid oxide fuel cell cathode comprising lanthanum nickelate.
This patent application is currently assigned to The Government of the US, as represented by the Secretary of the Navy. Invention is credited to Christel Laberty, Karen Swider Lyons, Anil V. Virkar, Feng Zhao.
Application Number | 20070184324 11/627485 |
Document ID | / |
Family ID | 38334452 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070184324 |
Kind Code |
A1 |
Lyons; Karen Swider ; et
al. |
August 9, 2007 |
SOLID OXIDE FUEL CELL CATHODE COMPRISING LANTHANUM NICKELATE
Abstract
A solid mixture of La.sub.2NiO.sub.4+.delta. and an ionic
conductive material. A solid oxide fuel cell having a cathode
interlayer having a La.sub.2NiO.sub.4+.delta. layer and a doped
ceria layer, a lanthanum strontium cobaltite or lanthanum strontium
manganate cathode current collector, an anode; and an ionic
conductive electrolyte between and in contact with the cathode
interlayer and the anode.
Inventors: |
Lyons; Karen Swider;
(Alexandria, VA) ; Laberty; Christel; (Paris,
FR) ; Zhao; Feng; (Salt Lake City, UT) ;
Virkar; Anil V.; (Salt Lake City, UT) |
Correspondence
Address: |
NAVAL RESEARCH LABORATORY;ASSOCIATE COUNSEL (PATENTS)
CODE 1008.2
4555 OVERLOOK AVENUE, S.W.
WASHINGTON
DC
20375-5320
US
|
Assignee: |
The Government of the US, as
represented by the Secretary of the Navy
Washington
DC
|
Family ID: |
38334452 |
Appl. No.: |
11/627485 |
Filed: |
January 26, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60762223 |
Jan 26, 2006 |
|
|
|
Current U.S.
Class: |
429/486 ;
429/496; 429/505; 429/519; 501/152 |
Current CPC
Class: |
H01M 4/9033 20130101;
H01M 2008/1293 20130101; C04B 35/488 20130101; H01M 8/0217
20130101; C04B 38/0074 20130101; H01M 8/04007 20130101; C04B
2111/00853 20130101; Y02E 60/50 20130101; H01M 8/04268 20130101;
C04B 35/48 20130101; C04B 38/0074 20130101; C04B 35/48 20130101;
C04B 35/488 20130101 |
Class at
Publication: |
429/033 ;
501/152; 429/026; 429/013 |
International
Class: |
H01M 8/12 20060101
H01M008/12; C04B 35/50 20060101 C04B035/50; H01M 8/04 20060101
H01M008/04 |
Claims
1. A composition of matter comprising a solid mixture of:
La.sub.2NiO.sub.4+.delta.; and an ionic conductive material.
2. The composition of matter of claim 1, wherein the ionic
conductive material is a rare earth oxide doped-ceria,
samaria-doped ceria, gadolinia-doped ceria, yttria-doped ceria,
ytterbia-doped ceria, dysprosia-doped ceria, holmia-doped ceria,
erbia-doped ceria, or terbia-doped ceria.
3. The composition of matter of claim 1, wherein the ionic
conductive material is yttria-stabilized zirconia.
4. The composition of matter of claim 1, wherein the ionic
conductive material is Sr-doped and Mg-doped LaGaO.sub.3.
5. The composition of matter of claim 1, wherein the composition
comprises from about 10 to about 90 wt % La.sub.2NiO.sub.4+.delta.
and from about 10 to about 90 wt % of the ionic conductive
material.
6. A solid oxide fuel cell cathode comprising: a cathode interlayer
comprising the composition of matter of claim 1; and a cathode
current collector comprising lanthanum strontium cobaltite or
lanthanum strontium manganate.
7. The solid oxide fuel cell cathode of claim 6, wherein the
cathode interlayer and the cathode current collector are porous
with contiguous porosity.
8. A solid oxide fuel cell comprising: the solid oxide fuel cell
cathode of claim 6; an anode; and an ionic conductive electrolyte
between and in contact with the cathode interlayer and the
anode.
9. The solid oxide fuel cell of claim 8, wherein the anode
comprises: an anode interlayer in contact with the ionic conductive
electrolyte; and an anode support in contact with the anode
interlayer.
10. The solid oxide fuel cell of claim 9, wherein the anode
interlayer and the anode support comprise porous nickel and
yttria-stabilized zirconia.
11. A method comprising: providing the solid oxide fuel cell of
claim 8; connecting an electrical load to the solid oxide fuel cell
cathode and the anode; supplying oxidant to the solid oxide fuel
cell cathode; supplying a fuel to the anode; and heating the solid
oxide fuel cell to a temperature sufficient to initiate reduction
of the oxygen and oxidation of the fuel.
12. The method of claim 11, wherein the temperature is at least
about 400.degree. C.
13. The method of claim 11, wherein the fuel is hydrogen.
14. A solid oxide fuel cell comprising: a cathode interlayer
comprising a La.sub.2NiO.sub.4+.delta. layer and a doped ceria
layer; a cathode current collector comprising lanthanum strontium
cobaltite or lanthanum strontium manganate; an anode; and an ionic
conductive electrolyte between and in contact with the cathode
interlayer and the anode.
15. The solid oxide fuel cell of claim 14; wherein the
La.sub.2NiO.sub.4+.delta. layer comprises at least about 95%
La.sub.2NiO.sub.4+.delta. and is from about 2 microns to about 40
microns thick; and wherein the doped ceria layer comprises a rare
earth oxide doped-ceria, samaria-doped ceria, gadolinia-doped
ceria, yttria-doped ceria, ytterbia-doped ceria, dysprosia-doped
ceria, holmia-doped ceria, terbia-doped ceria, or erbia-doped
ceria, and is at least about 2 microns thick.
16. The solid oxide fuel cell of claim 14, wherein the anode
comprises: an anode interlayer in contact with the ionic conductive
electrolyte; and an anode support in contact with the anode
interlayer.
17. The solid oxide fuel cell of claim 16, wherein the anode
interlayer and the anode support comprise porous nickel and
yttria-stabilized zirconia.
18. A method comprising: providing the solid oxide fuel cell of
claim 14; connecting an electrical load to the solid oxide fuel
cell cathode and the anode; supplying oxidant to the solid oxide
fuel cell cathode; supplying a fuel to the anode; and heating the
solid oxide fuel cell to a temperature sufficient to initiate
reduction of the oxygen and oxidation of the fuel.
19. The method of claim 18, wherein the temperature is at least
about 400.degree. C.
20. The method of claim 18, wherein the fuel is hydrogen.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/762,223, filed on Jan. 26, 2006,
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to compositions comprising
lanthanum nickelate and fuel cells made therefrom.
DESCRIPTION OF RELATED ART
[0003] Few materials exhibit the catalytic, electrical and
mechanical properties required for high activity and durability in
a high power density solid oxide fuel cells (SOFCs) cathode. Up to
60% of voltage loss in anode-supported SOFCs can occur at the
cathode, particularly at 800.degree. C. and lower temperatures, due
to polarization losses associated with the oxygen reduction
reaction (ORR) for the reduction of oxygen gas to oxygen ions,
O.sup.2- (Eq. 1)..sup.1 (All referenced publications and patent
documents are incorporated herein by reference.).
O.sub.2+4e.sup.-=2O.sup.2- (1)
[0004] State of the art SOFCs utilize porous composite cathodes
whereby the ORR primarily occurs at the three-phase-botuidary (TPB)
between an oxygen-ion conductor, an electronic conductor, and the
gas phase, to transport each species in Eq. 1. Considerable work
has been reported on cathode mechanisms and their dependence on
materials properties and microstructure, mainly via ex-situ
measurements and electrochemical characterization of individual
materials (half-cell measurements)..sup.1
[0005] Composite cathodes typically are a mixture of
strontium-doped lanthanum manganite (LSM), which has low electronic
resistance, but is a poor ionic conductor, and yttria-stabilized
zirconia (YSZ), which is a good oxygen-ion conductor, but has high
electronic resistance. Through optimization of the TPB and
porosity, high performance cells with LSM+YSZ-composite cathodes
have been developed. Important electrocatalytic properties of
composite cathodes are low electronic resistivity, .rho..sub.e, low
ionic resistivity, .rho..sub.i, low charge transfer resistivity,
.rho..sub.ct, combined with high TPB length, l.sub.TPB and
appropriate porosity, V.sub.v..sup.2 The highest perfortice of
LSM+YSZ-based cathodes, .about.1.9 W/cm.sup.2 at 800.degree. C.,
has been measured in SOFC button cells using "bi-layer" cathodes
wherein the LSM-YSZ composite is restricted to .about.20 .mu.m
thick "interlayer" in contact with the electrolyte where the ORR
occurs..sup.3 The mixed conducting cathode interlayer, or cathode
catalyst layer, is covered with a 50 .mu.m thick, low impedance LSM
"current collector." This is supported on an 8 to 10 .mu.m thick
YSZ electrolyte and a YSZ/Ni composite anode.
[0006] Most YSZ-based SOFCs are fabricated with LSM cathodes. The
perovskite structure of LSM has cation defects that facilitate
p-type electronic conductivity on the order of 100 Scm.sup.2 in air
at 700.degree. C. Additionally, LSM has good thermal and chemical
stability with the commonly used YSZ electrolyte and mechanical
strength at high temperatures. However, below 800.degree. C., the
overpotential of LSM electrodes on YSZ is significant, resulting in
low current densities. The electrochemical activity of LSM cathodes
has been improved by adding YSZ to improve mixed conduction of the
solid phase..sup.4 The oxygen-ion conductivity of the YSZ in
combination with the metallic LSM increases the TPB for the ORR.
The performance of the electrodes, as determined by their
impedance, can be very sensitive to the fabrication procedures,
which affect how the YSZ and LSM are interconnected..sup.4 It has
also been shown that the performance of the cell can depend
strongly on the microstructure and the geometry of the cell for a
given materials set..sup.3 Further, the performance of the cell can
be improved by using 5 layers of anode supported SOFC. For example,
the maximum power density achieved was 1.8 W/cm.sup.2 at
800.degree. C. and 0.4 W/cm.sup.2 at 600.degree. C. for an
optimized cell this given materials set (LSM, YSZ, Ni)..sup.3
SUMMARY OF THE INVENTION
[0007] The invention comprises a composition of matter comprising a
solid mixture of La.sub.2NiO.sub.4+.delta. and an ionic conductive
material.
[0008] The invention further comprises a solid oxide fuel cell
comprising: a cathode interlayer comprising a
La.sub.2NiO.sub.4+.delta. layer and a doped ceria layer; a cathode
current collector comprising lanthanum strontium cobaltite or
lanthanum strontium manganate; an anode; and an ionic conductive
electrolyte between and in contact with the cathode interlayer and
the anode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A more complete appreciation of the invention will be
readily obtained by reference to the following Description of the
Example Embodiments and the accompanying drawings.
[0010] FIG. 1 shows cell voltage and power density as a function of
the current density for a SOFC dime cell with a bi-layer cathode
having an LN and SDC interlayer and an LSC current collector and a
0.5 mm Ni/YSZ anode at 600, 700, and 800.degree. C. (Cell 6 in
Table 2).
[0011] FIG. 2 shows area specific resistance (ASR.sub.ohmic,
ASR.sub.total, vs. .DELTA.ASR.sub.polar) vs. measured maximum power
density at 800.degree. C. of Cells 1-6 from Table 2.
[0012] FIG. 3 shows a parametric fit of the polarization curve of
Cell 6 at 800.degree. C. (LN/SDC cathode interlayer and LSC current
collector). The fit is valid above about 0.5 A/cm where the Tafel
equation is applicable.
[0013] FIG. 4 shows a schematic of the single cell testing
apparatus used.
[0014] FIG. 5 shows voltage and power density vs. current density
plots for a porous (La.sub.2NiO.sub.4+.delta.+SDC) interlayer and
porous LSC current collector.
[0015] FIG. 6 shows the measured ohmic voltage loss as a function
of current density for a porous (La.sub.2NiO.sub.4+.delta.+SDC)
interlayer and porous LSC current collector.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0016] In the following description, for purposes of explanation
and not limitation, specific details are set forth in order to
provide a thorough understanding of the present invention. However,
it will be apparent to one skilled in the art that the present
invention may be practiced in other embodiments that depart from
these specific details. In other instances, detailed descriptions
of well-known methods and devices are omitted so as to not obscure
the description of the present invention with unnecessary
detail.
[0017] A long-term goal for improving SOFC cathodes has been to use
a single-phase mixed ionic-electronic conductor (MIEC) rather than
composites of ionically and electronically conducting ceramics, so
that the oxygen reduction reaction (ORR) occurs over the entire
surface of the cathode material rather than at the triple phase
boundary (TPB) between the composite and gas phases. Lanthanum
nickelate (LN, La.sub.2NiO.sub.4+.delta.) is one MIEC with
properties that should make it a good cathode catalyst. The LN
performs poorly (0.3 W/cm.sup.2) when it is used as a single-phase
cathode in yttria-stabilized-zirconia-based air-H.sub.2 button
cells at 800.degree. C. Power densities up to 2.2 W/cm.sup.2 are
measured only when LN is used in a composite bi-layer cathode. The
performance of the electrodes is fit with a parametric model for
composites. The results suggest that the ORR may preferentially
occur at a TPB, and composite electrodes are best for SOFC
cathodes.
[0018] A potentially promising path for further increasing the
cathode activity is to use single-phase, porous mixed
ionic-electronic conductors (MIECs) for the catalysts, whereby
there is a two-phase boundary between the ions and electrons in the
solid phase and oxygen gas, and the entire surface available for
the ORR rather than restricted to the TPB. Independent of
microstructural and electrical parameters, the 1 D macrohomogeneous
model for the impedance response of MIEC under zero-bias conditions
predicts that the chemical impedance is inversely proportional to
{square root over (D*k)}, where D* is the oxygen self-diffusion
coefficient and k is the oxygen surface exchange
coefficient..sup.5,6 Up to now, this model has been verified on
half-cell measurements with no imposed current.
[0019] Table 1 lists the electrical and kinetic properties of
various materials used in SOFC cathodes. High performance cathodes
have been made using the perovskites,
La.sub.0.7Sr.sub.0.3MnO.sub.3(LSM),.sup.3
La.sub.1-xSr.sub.xCo.sub.1-yFe.sub.yO.sub.3-d(LSCF)..sup.7 and
La.sub.1-xSr.sub.xCoO.sub.3-d(LSC)..sup.8,9 The MIEC model predicts
that D and k values equal to or greater than, respectively
10.sup.-8 cm.sup.2/s and 10.sup.-6 cm/s are needed to produce low
ASR values..sup.6 Although the D* and k values of LSM and LSCF are
apparently deficient, they are used in composites with ionic
conductors, for instance YSZ/LSM.sup.3,10 and LSCF/GDC.sup.6,8
whereby their D* and k values are reportedly 2 orders of magnitude
higher than their single constituents due to spillover
effects..sup.6,10 Table 1 indicates that LSC should be the best
cathode, with the highest D* and k values plus high electronic and
ionic conductivity. Unfortunately, LSC deleteriously reacts with
YSZ when sintered above 1000.degree. C., thereby prohibiting the
manufacture of a single-phase MIEC cathode SOFCs. LSC has been used
successfully in cathodes either by separation from the YSZ by a SDC
thin film, .sup.7 or infiltration into a porous YSZ network;.sup.9
both which are composite approaches, preventing the analysis of the
single LSC phase. TABLE-US-00001 TABLE 1 Properties of materials
used in SOFC cathodes including electronic resistivity, total
electrical resistivity, oxygen-ion resistivity, oxygen
self-diffusion coefficient, D*, and the oxygen surface exchange
coefficient, k, at 800.degree. C. and .gtoreq.0.21 atm. Where the
value has not been reported, it is estimated from analogous
compounds, designated as (est). Negligible values are designated as
(--). Electronic Total electrical Ionic resistivity resistivity
resistivity D* k Materials (.OMEGA. cm) (.OMEGA. cm) (.OMEGA. cm)
(cm.sup.2/s) (cm/s) Reference La.sub.0.7Sr.sub.0.3MnO.sub.3(LSM)
0.005 0.005 -- .sup. 4 .times. 10.sup.-15 1 .times. 10.sup.-8 10,
11 La.sub.0.7Sr.sub.0.3MnO.sub.3(LSM) 0.005 0.005 -- .sup. 4
.times. 10.sup.-15 1 .times. 10.sup.-8 10, 11
La.sub.0.7Sr.sub.0.3CoO.sub.3 (LSC) 0.003 0.003 500 (est) 2 .times.
10.sup.-6 2 .times. 10.sup.-5 12, 13
La.sub.0.6Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3 0.004 0.004 500
(est) 7 .times. 10.sup.-9 6 .times. 10.sup.-7 14, 15 (LSCF)
La.sub.2NiO.sub.4+.delta. (LN) 3 3 500 (est) 2 .times. 10.sup.-7 2
.times. 10.sup.-6 16, 17 Sm.sub.0.2Ce.sub.0.8O.sub.2 (SDC) -- 20 20
2 .times. 10.sup.-7 3 .times. 10.sup.-8 18 (est) (est)
Gd.sub.0.1Ce.sub.0.9O.sub.1.95 (GDC) -- 20 20 2 .times. 10.sup.-7 3
.times. 10.sup.-8 19, 20 YSZ -- 50 50 1 .times. 10.sup.-7 8 .times.
10.sup.-8 11, 19
[0020] Table 1 shows that the k of LN, 2.times.10.sup.-6 cm/s, is
an order-of-magnitude higher than that of LSCF and LSM. The value
of k decreases when Ni is replaced by Cu in the lattice,
La.sub.2CuO.sub.4+.delta. and when La is replaced with Sr to
increase the concentration of oxygen vacancies. These features are
correlated to the nature of the elements in site A and B and their
oxidation state. Shaw et al. have shown that the electrocatalytic
properties of A.sub.2BO.sub.4+d- type materials depend on the
cation, M, in the perovskite layer,.sup.21 and studies of the
(La,Sr)(Ni,M) systems have been shown that the oxygen diffusion is
also affected by the nature of the cation on the A site..sup.22
Electrochemical studies on these materials have shown that the
rate-determining step of the ORR under air is not due to oxygen
diffusion but surface processes, or k.
[0021] Compared to the perovskite-type oxides, the K.sub.2NiF.sub.4
structure accommodates a wide variety of oxygen stoichiometries.
Furthermore, excess oxygen can be incorporated in its interstitial
species, providing an attractive alternative to the vacancy-based
conduction mechanism present in perovskite or fluorite oxides. A
network of unoccupied interstitial sites exists when the oxygen
content is four and they become partially occupied when the
material oxidizes. For example, recent studies have shown that in
LN-based phases, the bulk ionic transport occurs via diffusion of
interstitial ions in the rock salt type layers and vacancies in the
perovskite layers of K.sub.2NiF.sub.4-type structure..sup.22,23
[0022] Electrical studies have shown that most LN-based materials
exhibit semi-conductor-like behavior in the range 300-750 K and an
apparent transition to pseudometallic behavior at higher
temperatures..sup.24 The transition results from oxygen loss on
heating, resulting in decreasing p-type carrier concentration; the
electronic-hole transport mechanism is via small polarons tip to
1250 K. The electronic conductivity reported for
La.sub.2NiO.sub.4-based materials is small (50 Scm.sup.-1) compared
to that of LSM oxide..sup.24 However, the LN conductivity can be
improved by doping with Sr..sup.25
[0023] Lanthanum nickelate, La.sub.2NiO.sub.4+.delta. or LN, with
the perovskite-related K.sub.2NiF.sub.4 (A.sub.2BO.sub.4+.delta.)
structure, exhibits a moderately low electrical resistivity of 3
.OMEGA.cm, a D*=2.times.10.sup.-7 cm.sup.2/s, and
k=2.times.10.sup.-6 cm/s. LN may have certain advantages over
perovskite LaNiO.sub.3 because of the high resistance of
LaNiO.sub.3 compared to LSCF, LSC, and LSM, and its loss of
performance due to formation of La.sub.2Zr.sub.2O.sub.7, NiO, and
La.sub.2NiO.sub.4 secondary phases over time..sup.26 The
K.sub.2NiF.sub.4 phase of LN is thermodynamically stable,.sup.27
and chemically and mechanically compatible with YSZ..sup.28 LN
performs well as an oxygen separation membrane, its performance
only surpassed by LSC among the materials in Table 1,.sup.29
conforming that it is a good MIEC. That LSC is a better MIEC oxygen
separation membrane than LN is supported by Table 1, which is
consistent with LSC having k and D* values an order of magnitude
higher than those of LN.
[0024] The activation energy of LN is from 0.6 -0.8 eV, which is
smaller than that of perovskites, presumably due to differences in
their conduction mechanisms. The oxygen-ions in the lanthanum
nickelates diffuse mainly by an interstitial mechanism in the
a.about.b plane,.sup.23 and Frenkel defects in the
c-direction..sup.30 Therefore, the ionic conductivity of
oxygen-hyperstoichiometric LN is equivalent to YSZ and 10 times
higher than that of oxygen deficient perovskites.
[0025] The thermal expansion coefficients (TECs) of the
A.sub.2BO.sub.4+.delta. materials also make them compatible with
common electrolytes such as YSZ, Sr-doped and Mg-doped LaGaO.sub.3
(LSGM), and gadolinia-doped ceria (CGO). The TEC of
La.sub.2Ni.sub.0.8Cu.sub.0.2O.sub.4+.delta. is linear from 400-1240
K with an average value of 13.3.times.10.sup.-6 .degree. C..sup.-1:
YSZ is .about.9.9.times.10.sup.-6 .degree. C..sup.-1 and LSGM and
Ce (Gd)O.sub.2-.delta. have values from 10.5-12.8.times.10.sup.-6
.degree. C..sup.-1..sup.25
[0026] The A.sub.2BO.sub.4+d materials appear to be chemically
stable. Nd.sub.2-xNiO.sub.4+.delta. does not react with YSZ and CGO
electrolytes after a heat treatment in air at 650.degree. C. over
five weeks..sup.22 However, heating some of these oxides in air at
800.degree. C. over five days, gives rise to the formation of
impurities such as Ln.sub.2Zr.sub.2O.sub.7 and LnNiO.sub.3-x (with
Ln=La, Nd).
[0027] The composition may comprise from about 10 or 20 to about 80
or 90 wt % La.sub.2NiO.sub.4+.delta. and from about 10 or 20 to
about 80 or 90 wt % of the ionic conductive material. Suitable
ionic conductive materials for use in the La.sub.2NiO4+.delta.
composition include, but are not limited to, a rare earth oxide
doped-ceria, samaria-doped ceria, gadolinia-doped ceria,
yttria-doped ceria, ytterbia-doped ceria, dysprosia-doped ceria,
holmia-doped ceria, terbia-doped ceria, erbia-doped ceria,
ytterbia-stabilized zirconia, scandia-stabilized zirconia,
yttria-stabilized zirconia, and LSGM.
[0028] A SOFC cathode may be made from a
La.sub.2NiO.sub.4+.delta.-ionic conductive material cathode
interlayer and a cathode current collector comprising lanthanum
strontium cobaltite or lanthanum strontium manganate. These layers
may be porous with contiguous porosity. This cathode may be
combined with an anode and an ionic conductive electrolyte between
and in contact with the cathode interlayer and the anode to form a
SOFC.
[0029] A suitable cell used in the present invention may consist of
the following five distinct layers: a) porous Ni+YSZ anode support,
b) porous Ni+YSZ anode interlayer, c) dense YSZ electrolyte, d)
porous (La.sub.2NiO.sub.4+.delta.+Sm.sub.2O.sub.3-CeO.sub.2)
cathode interlayer, e) porous La.sub.0.7Sr.sub.0.3CoO.sub.3-x
current collector.
[0030] The anode of the SOFC may comprise an anode interlayer in
contact with the ionic conductive electrolyte and an anode support
in contact with the anode interlayer. Suitable materials for these
layers include, but are not limited to, porous nickel and
yttria-stabilized zirconia.
[0031] An alternative construction of a SOFC uses a cathode
interlayer comprising a La.sub.2NiO.sub.4+.delta. layer and a doped
ceria layer with the cathode current collector, anode, and ionic
conductive electrolyte described above. The
La.sub.2NiO.sub.4+.delta. layer may comprise at least about 80 to
95% La.sub.2NiO.sub.4+.delta. and may be from about 2 or 20 microns
to about 40 microns thick. The doped ceria layer may comprise
samaria-doped ceria, gadolinia-doped ceria, yttria-doped ceria,
ytterbia-doped ceria, dysprosia-doped ceria, holmia-doped ceria,
erbia-doped ceria, or terbia-doped ceria and may be at least about
2 or 20 microns thick.
[0032] These SOFCs may be used by connecting an electrical load to
the cathode and the anode, supplying an oxidant, such as oxygen, to
the cathode and fuel to the anode, and heating the SOFC to a
temperature sufficient to initiate reduction of the oxygen and
oxidation of the fuel. Suitable fuels include, but are not limited
to, hydrogen, methane, and synfuel. The heating may be at least
about 400, 500, or 600.degree. C.
[0033] Having described the invention, the following examples are
given to illustrate specific applications of the invention. These
specific examples are not intended to limit the scope of the
invention described in this application.
Example 1
[0034] Synthesis of La.sub.2NiO4+.delta.--La.sub.2NiO.sub.4+.delta.
was synthesized Using a combustion technique adapted from the
literature. A solution was prepared with 0.95 M glycinec (98.5%,
Alfa) mixed with 0.3 M lanthanum(III) nitrate (99.99%, Alfa) and
0.15 M nickel(II) nitrates (99.9985%, Alfa) in 18 m.OMEGA.cm water.
The solution was evaporated on a hot plate from a beaker with
stirring at 90.degree. C. for several hours until it violently
ignited, leaving an amorphous, black powder. For safety, less than
100 mL in an 800 mL beaker was used for each combustion reaction,
and worked in a closed hood. The amorphous powder was ground in an
agate mortar and then heated in air at 5.degree. C./min to
1100.degree. C. and held for 2 h before cooling to room temperature
at 5.degree. C./min. The purity of the La.sub.2NiO.sub.4+.delta.
phase was confirmed by X-ray diffraction and matching to JCPDS
reference 34-0314. SEM (Leo Supra55) showed LN agglomerates less
than 1 .mu.m in diameter.
Example 2
[0035] Single cell testing--The suitability of LN as a SOFC cathode
has only been reported in ex situ tests and on "symmetrical cells"
with two air electrodes..sup.28,31 The objective of this example
was to confirm the suitability of LN as a SOFC cathode by
evaluating it in full "dime" cells under an air/H.sub.2 gradient.
The LN was evaluated as a stand-alone MIEC, and in bi-layer
composite cathodes. With this approach, measured performance was
interpreted based on available out of cell property
measurements.
[0036] Anode-supported SOFC "dime cells" 2.6 cm in diameter were
prepared at the University of Utah using previously developed
methods..sup.3 The anodes were 0.5 to 1 mm thick and the YSZ
electrolyte was 8 .mu.m thick. To make the cathode interlayer,
ethylene glycol slurries of LN, SDC (Praxair), or 50:50 LN:SDC were
ball milled with YSZ milling balls for 12 h. The final LN particle
size was 0.2 .mu.m, and the SDC particle size was about 0.5 .mu.m.
The slurries were painted in a 1 cm.sup.2 disk onto the YSZ
electrolyte, doctor-bladed to 25 .mu.m, dried on a hot plate, and
then heated in air at 5.degree. C./min to 1200.degree. C. and held
for 1 h. After firing, a thicker layer of LN, LSM (Praxair) or LSC
(Praxair) was applied by the same process and then was fired at
1150.degree. C. in air for 1 h. SEM of the cross-section of
fractured cells confirmed the thickness, porosity and adhesion of
each electrode layer.
[0037] The cell was mounted in single cell apparatus described
elsewhere where the cathode was exposed to open air, and the anode
chamber was formed with an alumina tube..sup.2 Contact to the cell
was made by spring loading between 1 cm.sup.2 silver mesh at the
cathode and 1 cm .sup.2 nickel mesh at the anode. Neither platinum
nor silver paste was used. The entire set-up was inserted into a
furnace and heated to 800.degree. C. with the anode under hydrogen
so that the NiO was reduced to Ni, resulting in a porous Ni-YSZ
composite anode with porosity of about 48%.
[0038] For testing, a typical cell was mounted in a test apparatus,
a schematic of which is shown in FIG. 4. This apparatus was
developed at the University of Utah to measure the performance of
single cell..sup.3 A fuel cell 10 was attached to two alumina
ceramics by multiple alumina tube supports 15. Fuel 20 and oxidant
25 were supplied through the ends of the alumina tubes. A
mechanical load was applied through an external spring 35 against
steel ring 40 to ensure good sealing. Silver foil and mica sheets
were used as gaskets. Silver mesh current collectors were attached
to cathode and anode. After assembly the entire set-up is inserted
into furnace to the desired operational temperatures. Electrical
circuit measures and/or uses the electrical current produced.
[0039] For electrochemical testing, the furnace was set to 600 to
800.degree. C., and the hydrogen was bubbled through water at room
temperature and fed into the cell at 300 mL/min; air was flowed at
550 mL/min to the cathode. The ASR was measured using two methods.
The first was the current interruption technique.sup.2 using a
Solartron S11287 Electrochemical Interface and Agilent 54622A
Digital Oscilloscope. The second was from slope of
.DELTA.V/.DELTA.I from the polarization curves over a range of
voltages.
[0040] The power densities and area specific resistance (ASR) of
SOFC button cells with six different cathode configurations are
listed in Table 2, with the polarization curves for cell 6 at 600,
700, and 800.degree. C. shown in FIG. 1 (H.sub.2:air=300 mL/min:550
mL/min; electrolyte thickness: 8 .mu.m). The ASR measured by
current interruption (CI) determines the total ohmic contribution
of the cell, or ASR.sub.ohmic. The ASR measured from the slope of
voltage (V) and current density (I) in the near linear regime of
the polarization curve represents the total resistance of the cell
from both ohmic and polarization losses (activation+concentration),
or ASR.sub.total. The difference of the two ASR values, or
ASR.sub.total-ASP.sub.ohmic, is thus a measure of the polarization
losses of the fuel cells, and is designated as
.DELTA.ASR.sub.polar. Table 2 lists the values for
.DELTA.ASR.sub.polar. FIG. 2 illustrates that the power density of
the cells was approximately inversely proportional to the
ASR.sub.total. In high performance cells (power densities>0.5
W/cm.sup.2), the power density was also approximately inversely
proportional to .DELTA.ASR.sub.polar with little influence from
ASR.sub.ohmic. Only Cells 1 and 2 with power densities<0.5
W/cm.sup.2 were affected by the ASR.sub.ohmic. In cells 3 and 4,
the dominant contribution was from .DELTA.ASR.sub.polar. However,
when the polarization resistance became very small, the ohmic
contribution began to dominate. Note that in cells 5 and 6, the
ohmic contribution was about the same as .DELTA.ASR.sub.polar.
TABLE-US-00002 TABLE 2 Power density at 0.7 V and 800.degree. C.,
Maximum power density at 800.degree. C., and ASR values at
800.degree. C. for different cathode compositions on anode
supported Ni-YSZ with YSZ electrolyte (9 .+-. 1 .mu.m thick) -
Anode support porosity: 48 vol % Cell 1 2 3 4 5 6 Current collector
(.mu.m) LMS -- -- -- 33 .+-. 1 -- -- LSC 50 .+-. 1 -- 48 .+-. 1 --
50 .+-. 1 50 .+-. 1 LN -- 20 .+-. 1 -- -- -- -- Interlayer (.mu.m)
SDC -- -- 13 .+-. 1 -- -- -- LN 17 .+-. 1 -- -- -- -- -- SDC + LN
-- 13 .+-. 1 -- 7 .+-. 1 20 .+-. 1 20 .+-. 1 Anode thickness
(.mu.m) 1000 1000 1000 1000 1000 500 Power density at 0.7 V and
800.degree. C. (W/cm.sup.2) 0.25 0.30 0.48 0.95 1.62 1.90 Max.
power density at 800.degree. C. (W/cm.sup.2) 0.30 0.40 0.60 1.2
1.96 2.20 ASR.sub.ohmic at 800.degree. C. (.OMEGA. cm.sup.2) 0.40
0.45 0.10 0.10 0.075 0.075 ASR.sub.total at 800.degree. C. (.OMEGA.
cm.sup.2) 1.3 0.94 0.61 0.31 0.19 0.18 .DELTA.ASR.sub.polar
(.OMEGA. cm.sup.2) 0.9 0.49 0.51 0.21 0.115 0.105
[0041] Despite predictions that LN would be a good catalyst layer,
Cell 1 with a LN-only interlayer and an LSC current collector
exhibited the lowest power density, 0.3 W/cm.sup.2, of the cells
tested. FIG. 2 clearly shows that this cell had the highest
.DELTA.ASR.sub.polar, indicating that the LN did not serve
effectively as a MIEC cathode catalyst and had poor kinetics for
oxygen reduction despite its reportedly high D* and k values. Cell
1 also had a high ASR.sub.ohmic, despite the low electronic
resistivity of both the LN and LSC (see Table 1). Because the LN
has higher electronic resistivity than LSC and LSM, it did not
serve well as a current collector. This was illustrated by Cell 2,
which had a low power density of 0.4 W/cm.sup.2, mainly due to its
high ASR.sub.ohmic, despite LN+SDC composite interlayer. Note that
a LN+SDC interlayer combined with a LN current collector also had
relatively high polarization losses.
[0042] Cell 3 demonstrated that SDC alone is also a poor interlayer
catalyst. It had a low ASR.sub.ohmic (0.1 .OMEGA.cm.sup.2) due to
the low ionic resistivity of SDC and low electronic resistivity of
its LSC current collector, but its high .DELTA.ASR.sub.polar
resulted in a maximum power density of only 0.6 W/cm.sup.2. These
results support that either a MIEC or a composite mixed
ionic-electronic conductor may be beneficial to ensure low
polarization losses or good kinetics for the ORR.
[0043] Similar to Cell 3, Cell 4 had an ASR.sub.ohmic=0.1
.OMEGA.cm.sup.2, but exhibited double the power density (1.2
W/cm.sup.2) due to a considerably lower .DELTA.ASR.sub.polar.
Unlike Cell 3, Cell 4 had a composite interlayer of LN and SDC, and
LSM current collector. The significant decrease in
.DELTA.ASR.sub.polar and increase in catalytic activity is
attributed to the implementation of this composite interlayer.
[0044] Cell 4 had a LSM current collector and a relatively thin
interlayer of 7 .mu.m (optimum is 20 .mu.m)..sup.2,32 As the cell
was improved in Cell 5 with a LSC current collector and a 20
.mu.m-thick LN+LSC interlayer, the maximum power density increased
by 60% to 1.96 W/cm.sup.2. These two changes caused only a slight
decrease in the ASR.sub.ohmic from 0.1 to 0.075 .OMEGA.cm.sup.2,
and most of the difference in the cell was the result of the
.DELTA.ASR.sub.polar decreasing from 0.21 to 0.12 .OMEGA.cm.sup.2.
The power density was increased finally in Cell 6 to 2.2 W/Cm.sup.2
by reducing the anode thickness to 500 .mu.m, causing little change
in the ASR.sub.ohmic but a decrease in the .DELTA.ASR.sub.polar to
0.105 .OMEGA.cm.sup.2, thus indicating that at these low levels,
some of the polarization losses are due to the anode.
[0045] The polarization curves for these LN-based cells were fit to
the parametric model developed earlier for composites of ionic and
electronics conductors..sup.2,3,33 FIG. 3 shows the fit. The
resulting effective gas phase diffusivities of the cathode
interlayer, D.sub.O.sub.2.sub.-N.sup.eff(2), cathode current
collector, D.sub.O.sub.2.sub.-N.sub.2.sup.eff(1), anode interlayer,
D.sub.H.sub.2.sub.-H.sub.2.sub.O.sup.eff(2), and anode support,
D.sub.H.sub.2.sub.-H.sub.2.sub.O.sup.eff(1) were 0.135, 0.04, 0.65
and 0.09 cm.sup.2/s, respectively. These values are nearly
comparable to those from LSM/YSZ cathode-based cells (0.14, 0.04,
0.68 and 0.08 cm.sup.2/s),.sup.3 indicating the microstructural
details were similar in the present cells. The fit of the ohmic
resistance, R.sub.i is 0.07.OMEGA.cm.sup.2, and compares well with
the ASR.sub.ohmic determined by current interruption
(0.075.OMEGA.cm.sup.2). Tafel values, a=0.08 and b=0.065, were
obtained for current densities above 0.5 A/cm.sup.2 for which the
Tafel law is applicable. The estimated exchange current density,
i.sub.0, 292 mA/cm.sup.2 was somewhat lower than the one observed
for LSMV/YSZ cells, (325 mA/cm.sup.2), despite the fact that the
LN/SDC cell exhibited higher power density..sup.3 This in part can
be attributed to the higher ohmic resistance for the LSM/YSZ cells
(0.104 .OMEGA.cm.sup.2). The indication is that with further
optimization of LN/SDC cathode-based cells, even higher performance
may be achievable.
Example 3
[0046] Temperature testing--FIG. 5 shows the performance of a cell
having a porous 10-20 .mu.m thick interlayer of
(La.sub.2NiO.sub.4+.delta.+SDC) and a porous 50 .mu.m thick current
collector of LSC tested at 600, 650, 700, 750, and 800.degree. C.
The maximum power density (MPD) was .about.1.96 W/cm.sup.2 at
800.degree. C., .about.1.5 W/cm.sup.2 at 750.degree. C., .about.1
W/cm.sup.2 at 700.degree. C., .about.0.55 W/Cm.sup.2 at 650.degree.
C., and .about.0.28 W/cm.sup.2 at 600.degree. C.
[0047] The results of the area specific ohmic resistance (ASR)
measured by current interruption on this cell are reported on FIG.
6 for various temperatures. As seen in this figure, the ASR
increased with decreasing temperature; the ASR increased from 0.076
to 0.4 .OMEGA.cm.sup.2 with the decrease of temperature from
800.degree. C. to 600.degree. C. The ASR increased from 0.085
.OMEGA.cm.sup.2 at 800.degree. C. to 0.15 .OMEGA.cm.sup.2 at
600.degree. C. for an optimized cell with the following materials
set: LSM, YSZ, Ni.
[0048] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that the claimed invention may be
practiced otherwise than as specifically described. Any reference
to claim elements in the singular, e.g., using the articles "a,"
"an," "the," or "said" is not construed as limiting the element to
the singular.
REFERENCES
[0049] 1. Adler, Chem. Rev., 104, 4791 (2004) [0050] 2. Tanner et
al., J. Electrochem. Soc., 144(1), 21 (1997) [0051] 3. Zhao et al.,
J. Power Sources, 141, 79 (2005) [0052] 4. Lee et al., J. Power
Sources, 115, 219 (2003) [0053] 5. Adler et al., J. Electrochem.
Soc., 144, 1884 (1997) [0054] 6. Steele et al., Solid State Ionics,
135, 445 (2000) [0055] 7. Jiang et al., J. Electrochem. Soc., 150,
A942 (2003) [0056] 8. Simner et al., J. Power Sources, 113, 1
(2003) [0057] 9. Armstrong at al., J. Electrochem. Soc., 153(3),
A515 (2006) [0058] 10. Ji et al., Solid State Ionics, 176, 937
(2005) [0059] 11. De Souza et al., Solid State Ionics, 106, 175
(1998) [0060] 12. Yamamoto et al., Solid State Ionics, 22, 241
(1987) [0061] 13. van Doorn et al., Solid State Ionics, 96, 1
(1997) [0062] 14. Chen et al., J Electrochem. Soc., 142, 491 (1995)
[0063] 15. Tai et al., Solid State Ionics, 76, 273 (1995) [0064]
16. Skinner et al., Solid State Ionics, 135, 709 (2000) [0065] 17.
Vashook et al., Solid State Ionics, 119, 23 (1999). [0066] 18.
Eguchi et al., Solid State Ionics, 52, 165 (1992) [0067] 19.
Manning et al., Solid State Ionics, 93, 125 (1997) [0068] 20. Huang
et al., J. Am. Ceram. Soc., 81, 357 (1998) [0069] 21. Shaw et al.,
Proc. 4th Eur. Forum, A. J. McEvoy, Editor, p. 611 (2000) [0070]
22. Boehm, Thesis, University of Bordeaux (2002) [0071] 23. Bassa
et al., 5.sup.th European solid oxide fuel cell forum, 1-5 Jul.
2002, Lucerne/Switzerland. ed. J. Huijsmans.
Oberrohrdorf(Switzerland): European Fuel Cell Forum, 2002. p.
586-593. ISBN 3-905592-10-X [0072] 24. Nishiyarna et al., Solid
State Communications, 94, 279 (1995) [0073] 25. Kharton et al., J
Mater. Chem., 9, 2623 (1999) [0074] 26. Ralph et al., J. Mater.
Sci., 36, 1161 (2001) [0075] 27. Demina et al., Inorg. Mater.,
41(7), 736 (2005) [0076] 28. M. L. Fontaine, Thesis, University of
Toulouse (2002) [0077] 29. Smith et al., J Electrochem. Soc., 153,
A233 (2006) [0078] 30. Minerviniet al., J. Mater. Chem., 10, 2349
(2000) [0079] 31. Mauvy et al., J. Electrochem. Soc, 153. A1547
(2006) [0080] 32. Virkar et al., Solid State Ionics, 131, 189
(2000) [0081] 33. Hu et al., J. Power Sources, 152, 22 (2005)
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